Hitherto, in order to contribute to miniaturization of portable equipment and achieving low power dissipation of the portable equipment, an oscillator is not mounted on the portable equipment as an independent part, and is formed on a substrate of a semiconductor device provided on the portable equipment together with other functional parts.
FIG. 6 is a circuit diagram of a differential oscillator formed on a semiconductor substrate as the conventional one.
A differential oscillator 100 shown in FIG. 6 has a first LC tank comprising an inductor 111_1 and a capacitor 112_1 one terminals of which are connected to a power supply VDD on a common basis and another terminals are connected to one another, and a second LC tank comprising an inductor 111_2 and a capacitor 112_2 one terminals of which are connected to a power supply VDD on a common basis and another terminals are connected to one another. The differential oscillator 100 further has an NMOS transistor 113_1 disposed between the connecting point of the inductor 111_1 and the capacitor 112_1 and the ground GND, and an NMOS transistor 113_2 disposed between the connecting point of the inductor 111_2 and the capacitor 112_2 and the ground GND. A gate of the NMOS transistor 113_1 is connected to the connecting point of the inductor 111_2 and the capacitor 112_2. A gate of the NMOS transistor 113_2 is connected to the connecting point of the inductor 111_1 and the capacitor 112_1.
In the differential oscillator 100, the first LC tank and the second LC tank are connected with the NMOS transistor 113_2 and the NMOS transistor 113_1 on a cross-coupling basis. Thus the differential oscillator 100 outputs oscillation signals I−, I+, which are mutually shifted by 180° in phase. The NMOS transistors 113_1 and 113_2 compensate for energy loss due to parasitic resistances of the inductors 111_1 and 111_2, and have sufficient gains for compensating for the energy loss. As a method of forming the inductors 111_1 and 111_2, there are proposed various forming methods. In the differential oscillator 100, the inductors 111_1 and 111_2 are formed on a silicon substrate using a standard process of CMOS. This brings about such an advantage that the manufacturing cost is inexpensive.
FIG. 7 is a view showing an inductor formed on a silicon substrate.
While FIG. 6 shows two inductors 111_1 and 111_2, FIG. 7 shows typically one of the two inductors 111_1 and 111_2 (which is referred to as an inductor 111).
Part (a) of FIG. 7 shows a top view of a helical inductor (an on-chip inductor) 111. Part (b) of FIG. 7 shows a sectional view of the inductor 111. As shown in the part (b) of FIG. 7, the inductor 111 is formed in such a manner that a helical conductor pattern 111a is disposed in an insulating layer 122 provided on a silicon substrate 121. The inductor 111 thus formed has a resistive component Rl involved in the helical conductor pattern 111a. Between the conductor pattern 111a and the silicon substrate 121, there exist capacitors 111b having parasitic capacitance Cs. On the silicon substrate 121, there exist substrate resistances 111c having resistance Rs.
The use of varactors or the like, which is a device having a so-called voltage control variable capacitance wherein a capacitance is varied in accordance with an applying voltage, as the capacitors 112_1 and 112_2 constituting the differential oscillator 100, makes it possible to implement a voltage controlled oscillator (VCO) for outputting an oscillation signal of an oscillation frequency according to a control voltage. While FIG. 6 shows an arrangement in which the one ends of the capacitors 112_1 and 112_2 are connected to the power supply VDD, in case of the voltage controlled oscillator, it is acceptable that the one ends of the capacitors 112_1 and 112_2 are connected to a terminal of a variable capacitance control signal.
FIG. 8 shows a small signal equivalent circuit to the oscillation signal I+ shown in FIG. 6.
In FIG. 8, vi denotes a small signal potential of the oscillation signal I+. The small signal equivalent circuit is expressed by a small signal current −vi gm generated by the gain gm when the oscillation signal I− is applied to a gate of the NMOS transistor 113_1, a capacitance C of the varactor, and an inductor encircled by a broken line, which comprises an inductance L, a resistance component Rl, a parasitic capacitance Cs, and a substrate resistance Rs. In case of the voltage controlled oscillator, in order to expand the range of the variable frequency, there is a need to maintain a capacitance other than the capacitance of the varactor small as much as possible. From this viewpoint it is important that the parasitic capacitance Cs is given with a small value as much as possible. In the event that the voltage controlled oscillator shown in FIG. 8 is used to perform an oscillation of a high frequency, setting up the resistance Rs of the silicon substrate disposed at the lower portion of the inductor to be large makes it possible to keep the influence of the parasitic capacitance Cs on the oscillation frequency small. In the event that the resistance Rs of the silicon substrate is sufficiently large, the equivalent circuit shown in FIG. 8 can be approximately replaced by an equivalent circuit shown in FIG. 9.
FIG. 9 is a view showing the equivalent circuit of the voltage controlled oscillator shown in FIG. 8 in the event that the resistance of the silicon substrate is sufficiently high.
According to the equivalent circuit of FIG. 9, the lower resistance component Rl of the inductor, the more it is possible to reduce the current necessary for the oscillation. Generally it is considered that the current necessary for the oscillation is proportional to a gain gs of a transistor necessary for the oscillation. Here, the gain gs of the transistor necessary for the oscillation is expressed bygm>(1/Rp)  (1)
where Rp is expressed byRp=Rl(1+Q2)  (2)
Where Q is expressed byQ=(ω0L/Rl)  (3)
Where the oscillation frequency ω0 is expressed byω0=(1/LC)1/2  (4)
Recently, as the radio communication transceiver technology developed, there is mounting necessity of an oscillator (a quadrature oscillator) for outputting two oscillation signals (those are referred to as I signal and Q signal) which are several GHz levels of high frequency and mutually shifted by 90 degrees in phase. Such an oscillator is incorporated into for example a down conversion section of a receiver, and is used as an image signal processor when a high frequency of radio signal is converted into a low frequency of radio signal.
FIG. 10 is a view showing the conventional quadrature oscillator.
A quadrature oscillator 110 shown in FIG. 10 is proposed in the publication “IEEE J. of Solid-State Circuits, April 1998 . . . Part 1; Architecture & Transmitter”. The quadrature oscillator 110 is provided with two differential oscillators 100 shown in FIG. 6. There are provided NMOS transistors 113_3 and 113_4 in parallel with NMOS transistors 113_1 and 113_2, respectively, which constitute the left side one of the two differential oscillators 100. Further, there are provided NMOS transistors 113_5 and 113_6 in parallel with NMOS transistors 113_1 and 113_2, respectively, which constitute the right side one of the two differential oscillators 100. Gates of the NMOS transistors 113_3 and 113_4 are connected to gates of the NMOS transistors 113_1 and 113_2, respectively, which constitute the right side one of the two differential oscillators 100. Gates of the NMOS transistors 113_5 and 113_6 are connected to gates of the NMOS transistors 113_2 and 113_1, respectively, which constitute the left side one of the two differential oscillators 100. The NMOS transistors 113_1 and 113_2 are each referred to as a differential type of loss compensating transistor. The NMOS transistors 113_3, 113_4, 113_5 and 113_6 are each referred to as a quadrature phase holding transistor. Voltages V(Q+), V(Q−), V(I+), and V(I−), which are represented by signals Q+, Q−, I+, and I− of the quadrature oscillator 110, respectively, are voltages which are mutually shifted by 90 degrees in phase as set forth below.V(Q+)=jV(I+)V(I−)=−V(I+)V(Q−)=−jV(I+)
FIG. 11 is a view showing a small signal equivalent circuit of the quadrature oscillator shown in FIG. 10 for the oscillation signal I+.
The small signal equivalent circuit is expressed by a small signal potential vi of the oscillation signal I+, a small signal current −vi gm á generated by the gain gm á when the oscillation signal I− is applied to a gate of the differential type of loss compensating transistor, a small signal current −jvi gm β generated by the gain gm β when the oscillation signal Q− is applied to a gate of the quadrature phase holding transistor, a capacitance C of the capacitor, and an inductor having an inductance L and a resistance component Rl. Incidentally, since the resistance Rs of the substrate disposed at the lower portion of the inductor is sufficiently high, the parasitic capacitance Cs and the substrate resistance Rs are omitted.
From this small signal equivalent circuit, the approximately same solution as the case of the above-mentioned differential oscillator 100 can be obtained.gm>(1/Rp)  (5)Rp=Rl(1+Q2)  (6)Q=(ω0L/Rl)  (7)ω0=(1/LC)1/2  (8)
As mentioned above, the oscillation frequency ω0 is expressed byω0=(1/LC)1/2,as given by equation (8), where the quadrature oscillatior 110 is driven by a small signal. In order to obtain a desired oscillation frequency ω0, an inductor having an inductance L which reflects the oscillation frequency ω0 is used. However, since the parasitic resistance component is involved in the inductor, there is a need to maintain the oscillation by flowing a current of the amount which reflects the parasitic resistance component. Thus, in the oscillator, an influence of the parasitic resistance component involved in the inductor upon the current necessary for maintaining the oscillation is large, and there is a problem with respect to reduction of the power dissipation of the oscillator.